1. Field of the Invention
The present invention is related to the processing of multi-colour images for reproduction on a printing or copying system. Particularly of interest are systems provided with at least five process colours for rendering these multi-colour images.
2. Discussion of the Related Art
Hereinafter, reference will be made to a multi-colour reproduction system being a multi-colour printing and/or copying system. In this disclosure, colour means all colours including black and white and all shades of grey.
In digital colour printing, continuous tones are rendered by halftoning the separation images in the process colours. The process colours are a limited number of colours of marking particles available on the multi-colour image reproduction system to render a colour image. Usually a distinction can be made among colour printing systems based on the kind of marking particles used, e.g. ink or toner, the imaging process employed, e.g. magnetography, or electro(photo)graphy, or inkjet, the productivity or the media range.
A distinction can however also be made dependent on the available number of process colours. The process colours correspond to the colours of the respective coloured marking particles available in the system such as e.g. black, white, cyan, magenta, yellow, orange, pink, red, green and blue. By selecting a number of process colours for a colour image reproduction system, one fixes the range of colours which can be produced by the colour image reproduction system, or in other words the gamut.
Most image reproduction systems employ the three classic chromatic colours: cyan (C), magenta (M) and yellow (Y), i.e. the so-called subtractive colours, and in most cases additionally black (K). The achievable gamut with the process colours of cyan, magenta, yellow and black is usually more restricted than the gamut of the image to be reproduced. To extend the gamut, more process colours need to be added. Typical process colour sets existing nowadays include sets of cyan, magenta, yellow and red, or of cyan, magenta, yellow, orange and green, or of the subtractive colours and red (R), green (G) and blue (B), the so-called additive colours, to extend the gamut respectively in the red, green and blue colours. To each of the afore-mentioned sets, the process colour black may be added.
When reproducing colour images, and particularly contone images, these images are processed by the colour image reproduction system such as to generate digital colour separation images in the process colours. The respective digital colour separation images are halftoned for enabling printing. Usually each colour separation image is halftoned using a different screen. A disadvantage of this approach employing a plurality of different screens, however, is its sensitivity for creating Moiré patterns. Moiré patterns are visible distortions in a rendered multi-colour image caused by interference patterns generated by combining halftone screens. Although it is known that the visible effect of Moiré patterns can be reduced by angling the halftone screens using predetermined screen angles, avoiding Moiré becomes particularly troublesome in colour image reproduction systems where four or more process colours can be printed. Thus in order to avoid Moiré the number of screens should be limited.
To meet this requirement, nowadays multi-colour image reproduction systems provided with the process colours yellow, magenta, cyan, red, green, blue and black exist wherein colour images are printed using the approach, for instance, as disclosed by Victor Ostromoukhov in “Chromaticity gamut enhancement by heptatone multi-color printing”, SPIE Proceedings 1993, Vol. 1909, pp.139-151. According to this approach, each pixel of a colour image is printed using a selection of at most three out of seven process colours, namely two chromatic colours: one additive primary colour (one of RGB) and one subtractive secondary colour (one of YMC) and an achromatic colour: black (K). As a consequence, in the heptatone printing process as disclosed by Ostromoukhov, the available gamut is divided in six sub-gamuts: KRY, KRM, KBM, KBC, KGC and KGY.
A first disadvantage of this approach is that a grey tone can only be printed with dots of black marking particles and thus, particularly at low image densities, the printed images are highly sensitive to graininess. Graininess is a perceived feature of a rendered colour which is among others related to how uniformly the coloured marking particles have been formed on the medium. Apart from image coverage, lightness differences and particularly unintentional variations in lightness differences determine to a large extent the amount of graininess. The higher the lightness differences are the more sensitive the printed images are with respect to graininess. Lightness differences include differences in lightness between non-overlapping dots of marking particles of different process colours, and in case superimposed multi-colour image reproduction systems are used, differences in lightness between non-overlapping dots of superimposed marking particles of different process colours. Usually the primary source for graininess is the lightness difference between the darkest colour, for instance black, and the lightest process colour present in the image such as for instance yellow.
A further disadvantage of the approach disclosed by Ostromoukhov is that each sub-gamut comprises only two chromatic process colours and thus a coloured pixel can only be printed with at most two chromatic process colours. Besides that the availability of only two chromatic process colours negatively affects graininess at low image densities, this also limits the ability for faithful colour reproduction of for instance photos and particularly e.g. image parts of light shades of pastel colours.